1. Field of The Invention
The present invention relates to a process for the formation of a cap layer for semiconductors, and more particularly to a process for the formation of a cap layer for semiconductors which is suitably used for a variety of semiconductors such as element semiconductors of Si (silicon) and Ge (germanium), and compound semiconductors of the group III-V or II-VI compounds, particularly the process is extremely effective for the III-V compound semiconductors.
2. Description of The Related Art
Heretofore, it has been known that since compound semiconductors of InGaAs quaternary mixed crystal system which lattice-match with InP (indium phosphide) can vary its bandgap from 1.35 eV to 0.72 eV, such compound semiconductors are applicable for long wavelength semiconductor laser device.
It has been known also that since InP has a high saturation drift velocity, such InP is applicable for ultrafast FET.
Moreover, it has been known that HEMT (High-Electron Mobility Transistor) using heterojunction of In0.52Al0.48As/In0.53Ga0.47As which lattice-matches with InP obtains such high band offset of 53 eV in conduction band, so that it is possible to realize ultrahigh speed operation due to formation of efficient two-dimensional electron gas (2DEG).
Thus, it is expected that a device prepared from InP or the related materials thereof can exhibit higher performance exceeding that of a device prepared from a material of GaAs (gallium arsenide) system as described above.
Meanwhile, no cap layer which can be easily formed on the surface of a semiconductor, and exhibits strong binding power and stability with respect to the surface of a semiconductor, besides only such cap layer can be selectively removed easily has been present in a variety of semiconductors, for example, compound semiconductors such as InP, and element semiconductors such as Ge unlike the case of xe2x80x9cSi in SiO2 (silicon dioxide)xe2x80x9d, and more specifically xe2x80x9cSiO2 being an oxide formed on the surface of Si as a cap layerxe2x80x9d.
Especially, it has been known that an oxide (i.e., an oxide of a component element of III-V compound semiconductors) formed on a surface of the III-V compound semiconductors such as InP brings about deterioration of electric characteristic properties such as increase in reverse leakage current, it is not desirable to use such oxide as a cap layer.
Therefore, in order to passivate a surface of such III-V compound semiconductors and to stabilize the same, a manner for passivating chemically the surface of the III-V compound semiconductors has been heretofore taken.
As a conventional manner for passivating a surface of III-V compound semiconductors, concerning a wafer of, for example, GaAs, GaP (gallium phosphide) or InP, xe2x80x9csulfur passivationxe2x80x9d wherein such wafer is treated in a solution containing S (sulfur) or a gas containing S (sulfur), whereby dangling bonds of a component element on a surface of the wafer are terminated with sulfur has been widely known.
However, the above described xe2x80x9csurface passivationxe2x80x9d involves such a problem which brings about pollution due to use of sulfur.
Furthermore, there have been also pollution problems of heavy metal contamination or disposal of spent solution (waste liquor).
Accordingly, it has been strongly demanded to provide a process for the formation of a cap layer for semiconductors which can be easily formed on the surface of a semiconductor, exhibits strong binding power and stabilization with respect to the surface of a semiconductor, and only such cap layer can be selectively removed easily, besides the cap layer has a low degree of pollution or contamination.
The present invention has been made in view of needs with respect to the above described prior art, so that an object of the invention is to provide a process for the formation of a cap layer for semiconductors which can be easily formed on the surface of a semiconductor, exhibits strong binding power and stabilization with respect to the surface of a semiconductor, and only such cap layer can be selectively removed easily, besides the cap layer has a low degree of contamination.
In order to achieve the above described object, the present invention has been constituted in such that nitrogen (N) which has strong binding power with (an)other element(s) as well as a low degree of contamination is introduced into the surface of a semiconductor, whereby a component element in the vicinity of the surface of the semiconductor is allowed to combine with nitrogen atom to form a nitride compound being a chemically passivated compound which is formed on the surface of the semiconductor, and such a nitride compound is utilized as a cap layer for the surface of a semiconductor.
The present invention as described above can be applied as a process for the formation of a cap layer for a general surface of an optical device or an electron device wherein, for example, a compound semiconductor is employed, and particularly the invention can be utilized as a process for the formation of a cap layer for preventing oxidation of a surface of a resonator in a semiconductor laser.
Namely, the present invention concerns a process for the formation of a cap layer for semiconductors comprising the steps of introducing nitrogen atom into a surface of a semiconductor; combining a component element of the semiconductor in the vicinity of the surface of the semiconductor into which the nitrogen atom has been introduced with the nitrogen atom to form a nitride compound being a compound of the component element of the semiconductor and the nitrogen atom on the surface of the semiconductor; and utilizing the nitride compound as a cap layer for the surface of the semiconductor.
Therefore, according to the present invention, since a cap layer of a nitride compound is formed by feeding only nitrogen onto the surface of a semiconductor, the cap layer can be very easily formed on the surface of the semiconductor, and in addition, nitrogen has a strong binding force and is stabilized with respect to other elements composing the semiconductor.
Moreover, only such cap layer of a nitride compound can be selectively removed from the surface of a semiconductor in accordance with such easy treatment that the cap layer is subjected to washing treatment by the use of an acid solution.
Since nitrogen is a material which does not become a cause of contamination unlike sulfur and the like, there is no fear of contamination, besides the formation of a cap layer can be carried out in accordance with a dry process without employing any solution, so that there is required no processing for a spent solution (waste liquor).
Furthermore, the present invention concerns a process for the formation of a cap layer for semiconductors comprising the steps of disposing a semiconductor in a gas containing nitrogen atom as a component element; heat-treating a surface of the semiconductor thereby to introduce the nitrogen atom contained in the gas into a surface of a semiconductor; combining a component element of the semiconductor in the vicinity of the surface of the semiconductor into which the nitrogen atom contained in the gas has been introduced with the nitrogen atom contained in the gas to form a nitride compound being a compound of the component element of the semiconductor and the nitrogen atom contained in the gas on the surface of the semiconductor; and utilizing the nitride compound as a cap layer for the surface of the semiconductor.
In the above described process, the heat treatment for the surface of the semiconductor may be such an instantaneous heat treatment that the surface of the semiconductor is instantaneously heated, and then cooled instantaneously, whereby a temperature in the surface of the semiconductor is maintained at a low temperature.
Further, the above described instantaneous heat treatment may be executed by irradiating pulse laser beam having a predetermined frequency and a predetermined fluence onto the surface of the semiconductor with a predetermined pulse number.
Still further, the above described semiconductor may be, for example, a III-V compound semiconductor.
Yet further, the present invention concerns a process for the formation of a cap layer for semiconductors comprising the steps of disposing, as a semiconductor, InP being a III-V compound semiconductor in ammonia gas being a gas containing nitrogen atom as a component element; heat-treating a surface of the InP in such a manner that pulse laser beam having a predetermined frequency and a predetermined fluence is irradiated onto the surface of the InP with a predetermined pulse number while maintaining a temperature in the surface of the InP at a low temperature thereby to introduce the nitrogen atom contained in the ammonia gas into the surface of the InP; combining In and P being component elements of the InP in the vicinity of the surface of the InP into which the nitrogen atom contained in the ammonia gas has been introduced with the nitrogen atom contained in the ammonia gas to form a thin film of InNx and PNx being a compound of In and P being the component elements of the InP and the nitrogen atom contained in the ammonia gas on the surface of the InP; and utilizing the thin film of the InNx and PNx as a cap layer for the surface of the InP.